1. Field of the Invention
This invention lies in the field of techniques for surface treatment of metal fluoride glass, particularly such glass which is suitable for the manufacture of fibers for optical communications technology.
2. Prior Art
Optical fibers based on silica glass which have their maximum transparency at the wavelength 1.5 .mu.m are presently employed in optical communications technology. For low attenuations, materials have been developed whose infrared absorption edge is shifted toward greater wavelengths and which are as good as, or better than, the known silica glass in terms of their scattering behavior. Halides particularly come into consideration here. In addition to the infrared absorption shift in the direction toward greater wavelengths by about 3-4 .mu.m in comparison to the oxides, it is particularly fluoride glasses which also exhibit adequate hydrolysis resistance. Whereas only fluoride glasses on the basis of highly toxic BeF.sub.2 were hitherto known, glasses of heavy metal fluorides which are less physiologically precarious can also be manufactured since 1975. The basic components herein are the tetravalent fluorides ZrF.sub.4, HfF.sub.4, ThF.sub.4 and their mixtures or the trivalent fluorides AlF.sub.3, ScF.sub.3 and YF.sub.3 as well as the bivalent fluorides such as, for example, CdF.sub.2. However, these main components only yield respective glass-forming melts in mixture with BaF.sub.2 (in this regard, see Marcel Poulain: Halide Glasses, J. Non-Cryst. Solids 56 (1983) 1-4).
Chemically, thus, it is a matter of barium salts of the complex fluoro-metallic acids. The crystallization tendency of the melts during cooling can be reduced by additives of chemically related fluorides, such as AlF.sub.3, LaF.sub.3, GdF.sub.3, PbF.sub.2, NaF or LiF. For transparent glasses, however, a rapid quenching is necessary, this only being achieved by means of a contact of the hot melt with a colder metal surface. This usually leads to samples which are more or less contaminated by foreign substances at their surface. Experience has taught that only fibers having low mechanical stability can be drawn from such glass rods obtained by means of mold casting.
A known method of resolving this problem is to mechanically polish the rods before the fiber-drawing and to thereby improve the stability (see, in this regard, H. Poignant, J. Le Mellot, Y. Bossis, "Infrared Fluorozirconate and Fluorohafnate Glass Optical Fibers", Proc. 8th Europ. Conf. Opt. Comm. Cannes, 1982, pp. 81-83). The hazard of contamination by grinding particles, however, thereby exists. Moreover, the glass surface easily becomes cloudy given contact with aqueous agents, this likewise having been described and being attributable to an ion interchange F.sup.- with OH.sup.- from the aqueous phase (see, in this regard, G. H. Frischat, I. Overbeck, "Chemical Durability of Fluorozirconate Glasses, J. Amer. Ceram. Soc. 67 (1984) C-238 --C-239). Measures for the removal of this cloudy layer have not yet been specified.